1. Field of the Invention
The present invention relates to the detection of noxious chemical species by means of a functionalized small molecule, i.e. non-polymeric sorbent compounds. More particularly, the invention relates to the detection of toxic or explosive chemical vapors, such as chemical agents or nitroaromatic species, by sorbent materials comprising small molecules with halogen substituted alcohol or phenol functional groups.
2. Description of Related Art
Determining and/or monitoring the presence of certain chemical species within an environment, e.g., pollutants, toxic substances and other predetermined compounds, is becoming of increasing importance with respect to such fields as health, environmental protection, resource conservation, and chemical processes. Devices for the molecular recognition of noxious species or other analytes typically include (1) a substrate and (2) a molecular recognition coating upon the substrate. These devices may be used, for example, in chemical vapor sensing or the selective separation of gases by gas chromatography. Small molecular recognition devices are described in Grate et al., Sensors and Actuators B, 3, 85–111 (1991) and Grate et al., Analytical Chemistry, Vol. 65, No. 14, Jul. 15, 1993, both of which are incorporated herein by reference.
Frequently, the substrate is a piezoelectric material or a waveguide, which can detect small changes in mass. One illustrative example of a device relying upon molecular recognition as a surface is known as a surface acoustic wave (SAW) sensor. SAW devices function by generating mechanical surface waves on a thin slab of a piezoelectric material, such as quartz, that oscillates at a characteristic resonant frequency when placed in a feedback circuit with a radio frequency amplifier. The oscillator frequency is measurably altered by small changes in mass and/or elastic modulus at the surface of the SAW device.
SAW devices can be adapted to a variety of gas-phase analytical problems by designing or selecting specific coatings for particular applications. The use of chemoselective polymers for chemical sensor application is well established as a way to increase the sensitivity and selectivity of a chemical sensor with respect to specific classes or types of analytes. Typically, a chemoselective polymer is designed to contain functional groups that can interact preferentially with the target analyte through dipole-dipole. Van der Waal's, or hydrogen bonding forces. For example, strong hydrogen bond donating characteristics are important for the detection of species that are hydrogen bond acceptors, such as toxic organophosphorus compounds. Increasing the density of available hydrogen bond acidic binding sites in the coating of a sensor results in an increase in sensitivity.
Chemoselective films or coatings used with chemical sensors have been described by McGill et al. in Chemtech, Vol. 24, No. 9, 27–37 (1994). The materials used as the chemically active, selectively absorbent layer of a molecular recognition device have often been polymers, as described in Hansani in Polymer Films in Sensor Applications (Technomic, Lancaster, Pa. 1995). For example, Ting et al. investigated polystyrene substituted with hexafluoroisopropanol (HFIP) groups for its compatibility with other polymers in Journal of Polymer Science: Polymer Letters Edition, Vol. 18, 201–209 (1980). Later, Chang et al. and Barlow et al. investigated a similar material for its use as a sorbent for organophosphorus vapors, and examined its behavior on a bulk quartz crystal monitor device in Polymer Engineering and Science, Vol. 27, No. 10, 693–702 and 703–15 (1987). Snow et al. (NRL Letter Report, 6210–884A) and Sprague et al. (Proceedings of the 1987 U.S. Army Chemical Research Development and Engineering Center Scientific Conference on Chemical Defense Research, page 1241) reported making materials containing HFIP that were based on polystyrene and poly(isoprene) polymer backbones, where the HFIP provided strong hydrogen bond acidic properties. These materials were used as coatings on molecular recognition devices, such as SAW sensors, and showed high sensitivity for organophosphorus vapors. However, both the parent polymers and the HFIP-containing materials were glassy or crystalline at room temperature. Because vapor diffusion is slowed in glassy or crystalline materials, the sensors produced were slow to respond and recover. Additional information is reported in Polym. Eng. Sci., 27, 693 and 703–715 (1987).
Grate et al. in Analytical Chemistry, Vol. 60, No. 9, 869–75 (1988), discloses a polymeric compound called “fluoropolyol” (FPOL), which is useful for detecting organophosphorus compounds. FPOL has the formula:
An HFIP-containing polymer based on a polysiloxane backbone was described and demonstrated to be a good hydrogen-bond acid by Abraham et al., “Hydrogen Bonding XXIX. The Characterisation of Fourteen Sorbent Coatings for Chemical Microsensors Using a New Solvation Equation”, J. Chem. Soc., Perkin Trans. 2, 369–78 (1995). The polysiloxane backbone provided a material with a Tg well below room temperature, however, other physical properties were not shown.
Grate, U.S. Pat. No. 5,756,631, discloses the use of hexafluoroalcohol-substituted siloxane polymers having the structure:
wherein R2 has the formula —(CH2)m-1—CH═CH—CH2—C(CF3)2—OH, and n is an integer greater than 1.
Grate et al., U.S. Pat. No. 6,015,869 discloses a strongly hydrogen bonding acidic, sorbent oligomer or polymer having a glass-to-rubber transition temperature below 25° C. The polymer has (1) fluoroalkyl-substituted bisphenol segments containing interactive groups and (2) oligodimethylsiloxane segments. These siloxane polymers are said to provide improved coatings and vapor sorption compositions for chemical sensors that are sensitive, reversible and capable of selective absorptions for particular vapors, particularly the hydrogen bond accepting vapors, such as organophosphorus compounds. However, these are still polymeric materials and, like all polymers, they can vary significantly from batch to batch in precise composition, purity and yield.
The inventors have now discovered a class of small molecules, rather than siloxane polymers, that can, in fact, be combined with polymeric matrices to produce composite hydrogen bond acidic coatings for chemical sensor applications. Using small molecules that are highly functionalized can result in significant sensitivity improvements. The host polymer matrix may or may not be functionalized with hydrogen bond acid groups. Thus, the functionalized small molecules described herein can improve the sensitivity of a sensor coating by increasing the density of hydrogen bond acid groups within the coating material.
Further, the chemoselective small molecules of the invention exhibit improved sensitivity to vapors of organophosphorus and nitroaromatic species, and are thus also useful for detecting the presence of these toxic materials. Conventional explosives, such as trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), may be contained in unexploded munitions, e.g., buried below the surface of the ground. Such munitions exude or leak vapors of the explosive. These vapors are typically concentrated in the surrounding soil and then migrate to the surface where they can be detected by the compounds, devices and methods of the invention. A similar situation exists for unexploded ordnance (UXO) found underwater.